Re: [PATCH] perf: write bpf_prog (infos|btfs)_cnt to data file

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Tue, Nov 11, 2025 at 02:38:07PM -0800, Namhyung Kim wrote:
> On Fri, Nov 07, 2025 at 11:31:50AM -0600, Thomas Falcon wrote:
> > With commit f0d0f978f3f58 ("perf header: Don't write empty BPF/BTF
> > info"), the write_bpf_( prog_info() | btf() ) functions exit
> > without writing anything if env->bpf_prog.(infos| btfs)_cnt is zero.
> > 
> > process_bpf_( prog_info() | btf() ), however, still expect a "count"
> > value to exist in the data file. If btf information is empty, for
> > example, process_bpf_btf will read garbage or some other data as the
> > number of btf nodes in the data file. As a result, the data file will
> > not be processed correctly.
> > 
> > Instead, write the count to the data file and exit if it is zero.
> 
> Oh, I'm afraid it'd create a compatibility problem.  But I think this is
> a right behavior and it's should be fine if it goes to the stable soon.
> 
> Arnaldo, can you please take this into perf-tools tree for v6.18?

Ok, agreed, I'm taking your request as an Acked-by, ok?

- Arnaldo
 
> Thanks,
> Namhyung
> 
> > 
> > Fixes: f0d0f978f3f58 ("perf header: Don't write empty BPF/BTF info")
> > Signed-off-by: Thomas Falcon <thomas.falcon@intel.com>
> > ---
> >  tools/perf/util/header.c | 10 ++--------
> >  1 file changed, 2 insertions(+), 8 deletions(-)
> > 
> > diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> > index db2ad19fa50d..54968881481c 100644
> > --- a/tools/perf/util/header.c
> > +++ b/tools/perf/util/header.c
> > @@ -1022,12 +1022,9 @@ static int write_bpf_prog_info(struct feat_fd *ff,
> >  
> >  	down_read(&env->bpf_progs.lock);
> >  
> > -	if (env->bpf_progs.infos_cnt == 0)
> > -		goto out;
> > -
> >  	ret = do_write(ff, &env->bpf_progs.infos_cnt,
> >  		       sizeof(env->bpf_progs.infos_cnt));
> > -	if (ret < 0)
> > +	if (ret < 0 || env->bpf_progs.infos_cnt == 0)
> >  		goto out;
> >  
> >  	root = &env->bpf_progs.infos;
> > @@ -1067,13 +1064,10 @@ static int write_bpf_btf(struct feat_fd *ff,
> >  
> >  	down_read(&env->bpf_progs.lock);
> >  
> > -	if (env->bpf_progs.btfs_cnt == 0)
> > -		goto out;
> > -
> >  	ret = do_write(ff, &env->bpf_progs.btfs_cnt,
> >  		       sizeof(env->bpf_progs.btfs_cnt));
> >  
> > -	if (ret < 0)
> > +	if (ret < 0 || env->bpf_progs.btfs_cnt == 0)
> >  		goto out;
> >  
> >  	root = &env->bpf_progs.btfs;
> > -- 
> > 2.47.3
> >